Traditional surveying typically includes two phases. In the first phase, the inclination and azimuth (which, together, essentially define a vector or unit vector tangent to the borehole) are determined at a discrete number of longitudinal points along the borehole (e.g., at a predetermined measured depth interval). Typically, no assumptions are required about the trajectory of the borehole between the discrete measurement points to determine inclination and azimuth. In the second phase, the discrete measurements made in the first phase are assembled into a survey of the well. In general, a particular type of well trajectory is assumed (e.g., the radius of curvature, tangential, balanced tangential, average angle, or minimum curvature assumptions are well known) and utilized to calculate a three-dimensional survey of the borehole. In recent years, the minimum curvature technique has emerged as an industry standard. This technique assumes that a circular arc connects the two measurement points. Referring to the two phases described above, the vectors measured in phase one are assumed to be tangential to the circular arc, and the arc is assumed to have a length equal to the difference in measured depth between the two points.
The use of accelerometers in conventional surveying techniques is well known. The use of magnetometers or gyroscopes in combination with one or more accelerometers to determine direction is also known. Deployments of such sensor sets are well known to determine borehole characteristics such as inclination, azimuth, positions in space, tool face rotation, magnetic tool face, and magnetic azimuth (i.e., an azimuth value determined from magnetic field measurements). While magnetometers and gyroscopes may provide valuable information to the surveyor, their use in borehole surveying, and in particular measurement while drilling (MWD) applications, tends to be limited by various factors. For example, magnetic interference, such as from magnetic steel or ferrous minerals in formations or ore bodies, tends to cause errors in the azimuth values obtained from a magnetometer. Motors and stabilizers used in directional drilling applications are typically permanently magnetized during magnetic particle inspection processes, and thus magnetometer readings obtained in proximity to the bottom hole assembly (BHA) are often unreliable. Gyroscopes are sensitive to high temperature and vibration and thus tend to be difficult to utilize in MWD applications. Gyroscopes also require a relatively long time interval (as compared to accelerometers and magnetometers) to obtain accurate readings. Furthermore, at low angles of inclination (i.e., near vertical), it becomes very difficult to obtain accurate azimuth values from gyroscopes.
U.S. Pat. No. 6,480,119 to McElhinney, hereafter referred to as the '119 patent, discloses a technique for deriving azimuth by comparing measurements from accelerometer sets deployed, for example, along a drill string. Using gravity as a primary reference, the '119 patent discloses a method for determining the change in azimuth between such accelerometer sets. The disclosed method assumes that the gravity sensor sets are displaced along the longitudinal axis of a downhole tool and makes use of the inherent bending of the tool between the gravity sensor sets in order to measure the relative change in azimuth therebetween.
Moreover, as also disclosed in the '119 patent, derivation of the azimuth conventionally requires a tie-in reference azimuth at the start of a survey section. Using a reference azimuth at the start of a survey results in subsequent surveys having to be referenced to each other in order to determine the well path all the way back to the starting tie-in reference. One conventional way to achieve such “chain referencing” is to survey at depth intervals that match the spacing between two sets of accelerometers. For example, if the spacing between the sets of accelerometers is 30 ft then it is preferable that a well is surveyed at 30 ft intervals. Optimally, though not necessarily, the position of the upper set will overlie the previous lower set.
While the borehole surveying techniques disclosed in the '119 patent are known to be commercially serviceable, considerable operator oversight and interaction is required to achieve high quality surveys. Furthermore, frequent calibration is often required during a survey to ensure data quality. It would therefore be highly advantageous to enhance gravity based surveying deployments so that such operator oversight and frequent calibration are not always necessary.